I suspect you didn't get any response to your first posting because you didn't specify what you meant by "service". It helps to be specific about what you need to know.
If you're simply talking about changing the diff fluid, there's a drain plug on the bottom and a fill plug on the side. If you do a search you'll find the procedure. Just make sure you loosen the fill plug before you remove the drain plug. The fill plug is pretty tight if its never been out and you don't want to drain the fluid just to find out you can't remove the fill plug!
As I recall, the fill plug requires a 14mm male hex adapter. Be sure to put the car on a secure lift or stands.
No, the diff fluid is NOT the same as the trans fluid. Check the books and get the right diff fluid.

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KenP

1996 S500 W140 Coupe